1. Field of the Invention
This invention relates to a projection type image display apparatus.
2. Related Background Art
With the arrival of the multimedia age, image display apparatuses are used in all scenes. Particularly, a projection type image display apparatus is easy to widen its image plane and therefore, front projectors have spread for presentation use, etc. and rear projectors have spread for home theater use, etc.
CRT's have heretofore been utilized as the light modulating elements of these projectors. In recent years, however, with the requirements for higher luminance and higher resolution, as light modulating elements suitable for these requirements, the utilization of liquid crystal panels and DMD's (digital mirror devices, see, for example, Japanese Patent Application Laid-Open No. 10-78550) is becoming the mainstream. A projection type image display apparatus utilizing the liquid crystal panel or the DMD illuminates the liquid crystal panel or the DMD as the light modulating element with light from a light source, and forms transmitted light or reflected light from the liquid crystal panel or the DMD into an image on a screen through a projection optical system.
Now, the projection type image display apparatus has not yet reached the quality of image of a direct view type CRT image display apparatus in high image quality display (display of which a feeling of quality is required). The high image quality (feeling of quality) referred to herein refers to a high dynamic range (high contrast and high gradation display). The direct view type CRT image display apparatus realizes a dynamic range of at least 10000:1 when it displays the entire image plane in black and white. On the other hand, the dynamic range of the projection type image display apparatus is determined chiefly by the characteristic of the light modulating element, and is about 300 to 400:1 in the case of the liquid crystal panel, and about 600 to 800:1 in the case of the DMD. This lowness of the dynamic range is attributable to the fact that in black display, in the case of a transmission type liquid crystal panel, leak light, and in the case of a reflection type liquid crystal panel or the DMD, scattered light is projected and a dark state floats.
So, in a projection type image display apparatus using a scattering type liquid crystal panel disclosed in Japanese Patent Application Laid-Open No. 7-84553, a large aperture diameter and a small aperture diameter are prepared in the stop of a projection optical system, and they are changed over in conformity with the kind of an input image to thereby make display improved in dynamic range possible.
However, the example shown in Japanese Patent Application Laid-Open No. 7-84553 suffers from the problem that the display improved in dynamic range is made possible, but luminance is lowered and the feeling of sharpness lacks.
Also, there has been actualized the problem that by the aperture diameter being made small, the brightness distribution of a projected image is varied (usually the marginal portion becomes relatively dark).